1. Field
The present disclosure relates generally to imaging systems and more particularly to imaging systems for use in nuclear medicine.
2. Description of Related Art
In a nuclear medicine imaging apparatus, a gamma or scintillation camera can obtain either planar images or Single Photon Emission Computed Tomography (SPECT) images, a collimator is mounted to the face of the imaging apparatus. The collimator is constructed of a dense, high-atomic-number material, such as lead. The material is bored with numerous tiny straight holes that allow radiation (e.g., gamma rays) to pass through. If the radiation is not traveling along the path of the hole, then the material absorbs it and it will not reach a detector of the nuclear imaging apparatus. The collimator thus collimates radiation before the radiation strikes a detector scintillation crystal. The radiation can be emitted from a distributed source (e.g., a radiopharmaceutical or radioisotope chosen for its affinity for a particular organ, tissue or region of the body) within a patient.
FIG. 2 is a block diagram of an exemplary SPECT or planar imaging apparatus. A radiation source 302 within an object to be imaged 304 (e.g., human body part) emits gamma photons that emanate from the object 304, pass through the collimator 308, and are captured by a scintillation detector 306, usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing, that converts the detected radiation into spatial incidence information. The system accumulates counts of gamma photons that are absorbed by the crystal in the detector 306. The crystal scintillates (i.e. emits low energy photons from the visible spectrum) in response to incident gamma radiation. An array of photomultiplier tubes (PMT) behind the crystal detects these fluorescent flashes and converts them into electrical signals. A computer 310 processes the electrical signals to determine energy and location of the incident photons and stores that incidence information along with other relevant information as the projection data. The computer 310 can then also display the projection data as two dimensional images of the relative spatial count density or distribution for each acquired view on a monitor. These images then reflect the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged. The individual two-dimensional images are also referred to as planar images because the depth information has been lost as they are taken from only one angle. The computer can then use projection data from a number of different viewing angles to reconstruct the volumetric (3d or 4d) distribution of the injected radioisotope, essentially restoring the lost depth information from the planar views by processing the plurality of planar views which have to fulfill certain tomographic criteria. This process is also called tomographic reconstruction.
A non-parallel hole collimator, such as a variable focus collimator, can be designed to magnify a specific organ onto the detector face yet to avoid truncation of the surrounding body. The non-parallel hole collimator thus allows for gain in count sensitivity, as measured by the reduction in dose or acquisition time needed to complete the clinical task with the same clinical sensitivity and specificity. Typically the target organ has to be at some specific distance to be in focal area depending on the collimator design and desired magnification. This distance is to be maintained to achieve better performance, reduce acquisition time and reduce dose. The key properties of such a collimator design has to be that it changes magnification both when object-collimator distance is changed, and when an object is moved parallel to the collimator surface. A simple fan-beam collimator is not suitable to achieve such requirements.
The variable focal collimator is used in cardiac imaging where a single organ is examined within the field of view (FOV) of the detector. The image of the single organ has a more or less enlarged projection image depending on the collimator properties and distance to the object. This magnification could provide an increase in sensitivity, allowing for the reduction of acquisition time and/or dose. Current methods of Planar or SPECT imaging do not provide sufficiently sensitive results, particularly when scanning over a large area, such as an entire body in a scan of the skeletal system as it is done in oncology imaging. More sensitive results would result in more accurate diagnosis of pathology such as tumors. To obtain sensitive images, multiple scans can be made using the same or different imaging apparatuses, which can require a patient to move from one apparatus to another which causes delay, increases costs, is cumbersome, and has lower specificity and sensitivity. Additionally, the multiple apparatuses are not optimized and therefore still result in lower image quality.